Implantable medical devices (IMD's), for example, cardiac pacemakers, defibrillators, neurostimulators and drug pumps, which include electronic circuitry and battery elements, require a housing to contain and hermetically seal these elements within a body of a patient. Many of these IMD's include one or more electrical feedthrough assemblies to provide electrical connection between the elements contained within the housing and components of the IMD external to the housing, for example, sensors and/or electrodes and/or lead wires mounted on an exterior surface of the housing, or electrical contacts housed within a connector module, which is mounted on the housing to provide coupling for one or more implantable leads, which typically carry one or more electrodes and/or one or more other types of physiological sensors. A physiological sensor, for example a pressure sensor, incorporated within a body of a lead may also require a hermetically sealed housing to contain electronic circuitry of the sensor, and, thus an electrical feedthrough assembly to provide electrical connection between one or more lead wires, that extend within the implantable lead body, and the contained circuitry.
A feedthrough assembly typically includes one or more feedthrough pins that extend from an interior to an exterior of the housing through a ferrule; each feedthrough pin is electrically isolated from the ferrule, and, in the case of the multipolar assembly, from one another, by an insulator element, for example, glass or ceramic, that is mounted within the ferrule and surrounds the feedthrough pin(s). Glass insulators are typically sealed directly to the pin(s) and to the ferrule, for example, by heating the assembly to a temperature at which the glass wets the pin(s) and ferrule, while ceramic insulators are typically sealed to the pin(s) and to the ferrule by a braze joint.
Special glass compositions, which are suitable for use as insulators in implantable feedthrough assemblies are disclosed in commonly-assigned U.S. Pat. No. 6,090,503 and in commonly-assigned and co-pending U.S. patent application 2006/0247714, which are both hereby incorporated by reference; one example of these glasses is known as CABAL-12 glass (developed by Sandia National Laboratories). With reference to the table in FIG. 11 of patent application '714 it may be appreciated that many of these glasses have a relatively high boron oxide content. The boron oxide in these glasses can provide resistance to corrosion by lithium (relevant to battery feedthrough assemblies), and can enhance a reliability of seals created with titanium, for example, used for either or both of feedthrough pins and ferrules. However, the relatively high boron content makes the glasses hygroscopic, so that performance of these types of glasses as a feedthrough insulator in an aqueous environment, for example, as implanted in a human body, has been placed under scrutiny.